As is known, in a hydrofoil seacraft of the submerged foil type, the hull of the craft is lifted out of the water by means of foils which are carried on struts and pass through the water beneath the surface thereof. In passing through the water, and assuming that sufficient speed is attained, the foils create enough lift to raise the hull above the surface and, hence, eliminate the normal resistance encountered by a ship hull in passing through the water.
In the usual case, there are forward and aft foils, both provided with control flaps similar to those used on aircraft. The other essential element is the rudder which pierces or is submerged beneath the surface of the water and is either forward or aft of the craft, depending upon its design. In most hydrofoils, the flaps are used primarily to cause the craft to ascend or descend and to control the craft about its pitch and roll axes; however they can also be used in combination with the rudder to bank the ship about its roll axis during a turn.
Steering and holding against the moments generated by beam winds had been a continual problem in hydrofoil craft until the fully-swiveled strut for steering was introduced. In such an arrangement, a strut, usually the forward strut, is pivoted about an essentially-vertical axis and pivots along its entire length to effect a turn. The fully-swiveled strut has been used extensively, and has effectively eliminated any serious problems in steering and maintaining positive directional control of hydrofoil craft.
The degree of improvement in directional control for such a hydrofoil craft with the use of a fully-swiveled strut, however, has not been attained without some penalties. That is, the fully-swiveled strut involves added costs, added weight, and added complexity. Moreover, as hydrofoils become larger, the costs and complexities associated with the fully-swiveled strut become greater and greater. In some present-day hydrofoils, for example, the strut alone weighs about 7.8 long tons. Consequently, the implementation of a means for actuating the strut under the forces encountered during a turn becomes increasingly difficult.
Prior attempts at steering a hydrofoil craft without using a swiveled strut involved the utilization of trailing-edge rudders on a fixed strut, spade rudders below the foils, forced-air ventilation of the strut to generate side forces without trailing-edge or spade rudders, and various other methods associated with ventilation of one side or the other of the strut. These attempts have all been singularly unsuccessful in that they have not been able to provide smooth, positive control forces. As a result, they are not employed on present-day hydrofil ships of the fully-submerged foil type.